Abstract: By weld thermo-simulation test with different peak temperatures and t_8/5,the variation of HAZ microstructures of 10Ni5CrMoV steel was studied.The metallographic observation revealed that when the peak temperature was between Ac₁~Ac₃,a mixture microstructure with fine martensite and tempering martensite of high temperaturewas obtained, moverover auto-tempering martensite was produced. When the peak temperature over Ac₃,the HAZ microstructures were completely trans formed to martensite after cooling. The higher the peak temperature, the coarser the grain size of prior austenite, the larger the martensitic lath bundle after phase transformation.It was also found that with t_8/5 increasing the microstructure change from auto-tempering martensite (containing a little transformation twin),auto-tempering martensite+LB,auto-tempering martensite+LB+UB, LB+UB, to the final microstructure in which UB vasts majority was formed.